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+=head1 NAME
+
+base - Establish IS-A relationship with base class at compile time
+
+=head1 SYNOPSIS
+
+ package Baz;
+ use base qw(Foo Bar);
+
+=head1 DESCRIPTION
+
+Roughly similar in effect to
+
+ BEGIN {
+ require Foo;
+ require Bar;
+ push @ISA, qw(Foo Bar);
+ }
+
+Will also initialize the %FIELDS hash if one of the base classes has
+it. Multiple inheritance of %FIELDS is not supported. The 'base'
+pragma will croak if multiple base classes have a %FIELDS hash. See
+L<fields> for a description of this feature.
+
+When strict 'vars' is in scope I<base> also let you assign to @ISA
+without having to declare @ISA with the 'vars' pragma first.
+
+If any of the base classes are not loaded yet, I<base> silently
+C<require>s them. Whether to C<require> a base class package is
+determined by the absence of a global $VERSION in the base package.
+If $VERSION is not detected even after loading it, <base> will
+define $VERSION in the base package, setting it to the string
+C<-1, set by base.pm>.
+
+=head1 HISTORY
+
+This module was introduced with Perl 5.004_04.
+
+=head1 SEE ALSO
+
+L<fields>
+
+=cut
+
+package base;
+
+use 5.006_001;
+our $VERSION = "1.03";
+
+sub import {
+ my $class = shift;
+ my $fields_base;
+ my $pkg = caller(0);
+
+ foreach my $base (@_) {
+ next if $pkg->isa($base);
+ my $vglob;
+ if ($vglob = ${"$base\::"}{VERSION} and *$vglob{SCALAR}) {
+ $$vglob = "-1, set by base.pm" unless defined $$vglob;
+ } else {
+ eval "require $base";
+ # Only ignore "Can't locate" errors from our eval require.
+ # Other fatal errors (syntax etc) must be reported.
+ die if $@ && $@ !~ /^Can't locate .*? at \(eval /;
+ unless (%{"$base\::"}) {
+ require Carp;
+ Carp::croak("Base class package \"$base\" is empty.\n",
+ "\t(Perhaps you need to 'use' the module ",
+ "which defines that package first.)");
+ }
+ ${"$base\::VERSION"} = "-1, set by base.pm" unless defined ${"$base\::VERSION"};
+ }
+ push @{"$pkg\::ISA"}, $base;
+
+ # A simple test like (defined %{"$base\::FIELDS"}) will
+ # sometimes produce typo warnings because it would create
+ # the hash if it was not present before.
+ my $fglob;
+ if ($fglob = ${"$base\::"}{"FIELDS"} and *$fglob{HASH}) {
+ if ($fields_base) {
+ require Carp;
+ Carp::croak("Can't multiply inherit %FIELDS");
+ } else {
+ $fields_base = $base;
+ }
+ }
+ }
+ if ($fields_base) {
+ require fields;
+ fields::inherit($pkg, $fields_base);
+ }
+}
+
+1;